Polyesters are commonly used for blow-molding oriented hollow articles such as beverage bottles and other containers. It is commercially important to be able to form containers having uniform sidewall thickness. A typical blow-molding manufacturing process involves 1) molding polymer pellets into specially shaped parisons; 2) extruding the parison to form a hollow polymer tube of uniform side wall thickness; 3) clamping a mold having the desired finished shape around the extrudate, and 4) blowing air into the hollow extrudate, causing the extrudate to stretch and expand to fill the mold. In order to form containers having uniform sidewall thickness, the polymer extrudate must maintain dimensional stability without excessive sag.
In order to achieve this objective, blow-molding facilities have 1) used polyesters exhibiting the best known melt strengths and 2) increased the rate of the blow-molding process. Melt strength is the ability of a material to maintain dimensional stability while in the softened or molten state. A material having high melt strength has a tendency to resist stretching and flowing as a result of gravitational force when in the softened or molten state. Melt strength has generally been determined by measuring the vertical length of an extrudate after a certain amount of time. Shorter lengths indicate better melt strength and, consequently, less sag.
Polyesters such as polyethylene terephthalate, poly(ethylene-co-1,4-cyclohexanedimethanol) terephthalate, and branched polyesters have been used in blow molding. The polyesters found most useful in extrusion blow molding operations have high melt strengths due to unusually high molecular weights. However, the utility of the ultra high molecular weight polymers is limited due to processing difficulties associated with their high melt viscosities and molecular weights. The use of branched polymers is a technique known for providing polymers which have been found to have adequate melt strengths for blow molding operations. However, branched polymers are often difficult to synthesize consistently.
Poly((70/30)ethylene-co-1,4-cyclohexanedimethanol) terepthalate is a relatively low viscosity linear polymer which has been widely used in blow-molding. It is more easily processed than polymers having branching or ultra high molecular weights.
The polyesters known to be useful in the art for blow-molding applications exhibit poor to moderate melt strength and sag. Therefore, commercial blow-molding has been limited to procedures that can be carried out relatively quickly so that the polymer material used does not significantly change in dimensional structure. The use of a relatively low viscosity polymer having good melt strength and good (low) sag would enable more latitude in developing blow-molding techniques for different products.
In light of the above it would be very desirable to provide polyesters having good melt strength and low sag. It would be especially useful for such polyesters to be easily machinable and melt processable. Such polyesters would be especially useful in the production of oriented articles, containers, bottles, fibers, films, and sheeting.